1. Field of the Invention
The present invention relates to a method of producing and treating glass threads and, more particularly, to a method of producing and treating glass threads including a step in which the glass threads are dried after the size has been deposited, as well as to a device for performing this method.
2. Description of the Related Art
In glass thread production, thin streams of glass are drawn jointly, in the form of one or a plurality of fans of filaments, through a die plate from a mass of molten glass and the filaments are then covered with a coating known as "size" intended to impart to them specific properties essential for subsequent handling or transformation operations, this occurring before filaments are gathered together in one or a plurality of basic threads. These threads can then be wound onto rotating supports before being subjected to other operations typically associated with organic threads, or even cut into sections after formation.
The drawing speeds are generally fast (several tens of meters per second), the number of filaments high (several hundred to several thousand) and their diameter small (a few microns). The application of size, intended to protect the threads against abrasion and connect the filaments while imparting the above properties to them, must thus be suitable for these glass thread production systems. However, it is difficult to control the amount of size retained by the threads, which frequently results in an excess of size being deposited as a precautionary measure. The size composition contains a large proportion of generally aqueous solvent which has to be eliminated, whereas the active compounds present in the size are fixed on the threads. But the elimination process is frequently poorly controlled or too late. Some of the solution is lost at the gathering device as a result of friction or during winding as a result of centrifugal force. Residual moisture, generally on the order of 10% by weight, nonetheless remains which should be removed to a greater or lesser extent depending on manufacturing requirements and the applications for which the threads are intended.
Particularly when the threads are in the form of windings, some methods provide for drying by stoving when the threads have been produced. However, as well as involving an additional operation, these methods pose the problem of the size migrating within the windings. Indeed, the aqueous solution moving towards the periphery of the windings before it evaporates entrains the active components of the coating such that the final concentration of size is far greater on the parts of the threads at the periphery of the winding that on the parts of the threads located at the center of the winding. Once the ends of the threads have been unwound, they are useless since there is too great a disparity between their properties and they have to be removed, which involves further waste of material and time. In addition, owing to the fact that the components of the size solution have a variable affinity with glass, the migration process occurs selectively, the effect being that the composition of the size solution covering the threads in the winding varies with respect to the composition of the size as initially deposited on these same threads. Thus, for these two reasons, this migration phenomenon results in irregular thread windings, leading to products which are not homogeneous and which are, consequently, unsatisfactory.
In addition to the migration phenomena, there are sometimes also phenomena of coloration of the threads at the periphery of the windings, these phenomena being caused by the use of certain gases within the ovens, as well as phenomena of deformation of the windings during stoving under the effect of steam pressure when there is a large amount of water to be removed. In both cases, the threads which have been modified in this way cannot be used.
It is also known to dry the threads directly below the sizing device using a microwave cavity (U.S. Pat. No. 3,653,860). These processes are rapid and efficient but are only useful for threads with a low count (less than 450 tex). Indeed, the power which can be used within these cavities is limited, firstly, by the power of the generator used and, secondly, due to the electrical discharges which occur above a given power level and which are due to the ionization of the air and to humidity. Thus, it is impossible to dry all types of thread and, in particular, rovings with a high count.
It is further known to dry the filaments before the gathering comb, using the air heated by the die (WO Patent 92/05122). This air is entrained by the moving filaments and is channeled to the point at which the filaments are gathered together. Here again, a process of this type is of limited application since it does not permit adaptation according to the type of thread produced. Indeed, it is impossible to control the amount of air used or its temperature, said amount being imposed by the drawing speed and number of filaments, and the temperature produced by the die.